Coal accounts for about 70 percent of world energy reserves, while oil and natural gas together account for about 12 percent. Thus, the research and development of coal utilization technology is one of the important parts of energy strategy. However, with the emergence and development of petrochemical industry, the proportion of coal in chemical raw materials has been declining and gradually replaced by oil and natural gas. Until the late 1970s, the development of global petrochemical industry was affected by the soaring oil price. Meanwhile remarkable progress was made in coal gasification and coal liquefaction of coal chemical industry. Especially after the 1990s, world oil prices remained at a high level for a long time and showed a rising trend, which further promoted the development of coal chemical technology and people again attach importance to coal chemical industry. Chinese nonrenewable resources reserves are characterized by being rich in coal, poor in oil and short in gas, and as an essential part of Chinese energy resources, low-rank coal like lignite and long flame coal account for more than 30 percent of Chinese coal reserves, thus the gasification of low-rank coal is a key research field for China.
The structural characteristics of coal determine the high volatile component and strong activity of low-rank coal. Because of the high proportion of moisture and oxygen and low heat value, direct utilization like combustion or gasification has low efficiency and the economic value is far less than high-rank coal. Thus, it is essential to process the low rank coal to improve its quality before the large-scale development and utilization. One of the most scientific and commonly used methods is pyrolysis, namely, “dry distillation” or thermal decomposition. Pyrolysis refers to a series of physical changes and chemical reactions that occur in coal when it is continuously heated to a higher temperature in the absence of air or in inert gases. In this process, the fracture of cross link bond, recombination of products and the secondary reaction will occur, gas(coal gas), liquid(tar), solid (semicoke) generate eventually. Pyrolysis is an efficient comprehensive resource utilization method that achieves the cascade conversion of different components in coal. It has the advantage of reducing the environmental pollution caused by coal combustion and increasing the comprehensive utilization value of low-rank coal, which can create significant economic and social benefits. However, due to the characteristics of low-rank coal, a large amount of gaseous tar, most of the volatile component and moisture generated in pyrolysis will enter the gasification gas together. Excessive tar easily causes precipitation and clogs the pipe. Using purified gasification gas need a lot of water for washing and cooling, resulting in a large amount of wastewater, which is difficult and expensive to treat, and direct emissions will cause serious pollution to environment. Similar problems also exist in the field of biomass gasification.
Therefore, how to provide a solid fuel gasification apparatus that can not only accomplish effective removal of tar and zero discharge of coking wastewater, but also optimize gas quality during the process, improve energy efficiency and reduce operating cost becomes an industry-wide problem in need of solution.